A new type of smart window, with interesting aesthetic and energy saving characteristics, can be achieved by the use of electrochromic (EC) materials. With application of a very low DC bias (1˜2 V) the electrochromic device changes its absorption bands and from a transparent state it turns into a colored state. In this state the device absorbs a large amount of the impinging radiant energy and just a very little fraction of radiation is transmitted. The resulting device is almost transparent and can change color by application of a few DC Volts.
Such a window therefore changes from clear to dark, blocks glare, stops solar heat and never obstructs the view. It consists of a series of thin conducting layers that change optical properties persistently and reversibly when an electrical voltage is applied. One layer called Prussian blue acts as a positive electrode and another layer of tungsten oxide acts as a negative electrode and a ion conducting electrolyte separates the two layers. When voltage is applied, cations stored in the positive electrode, traverse to the negative electrode, a process that turns transparent tungsten oxide to tungsten metal bronze (a light absorbing—blue layer) formed by the chemical addition of ions. Simultaneously, the anodic Prussian blue layer undergoes oxidation and acquires a deep blue color, which is complementary to the color change experienced by the cathodic tungsten oxide film. The longer the duration, for which the voltage is applied, more ions are transferred and the window becomes darker. It is important to recall that EC films can block out the visible as well as the infrared part of the solar spectrum and can therefore reduce costs associated with heating, lighting and cooling.
Once the applied voltage is reversed, the process is also reversed, these windows use energy only to change their optical state and no continuous power is required to maintain any desired particular optical state. If the voltage is turned off while switching, the glass retains the color attained. This is attributed to the battery-like behavior of the EC device. Energy consumption for one full coloring cycle, or for minimum to maximum bleaching, is approximately 200 W/m2.
The response times vary from a few seconds to a few minutes depending upon the size of the window. The technology can save substantial amounts of energy in buildings/automobiles and dynamic electrochromic glazings may eventually replace traditional solar static control technologies such as tints, reflective coatings and shading devices.
Although research in this area dates back to the 1960s, no reliable large-area electrochromic (EC) product for smart window applications has been brought to market. This is mainly due to issues involving cost, performance, and the stability of prospective devices and production methods.
In prior art, attempts relating to reversible mirror devices that employ conventional organic solvents have been described in, for example, U.S. Pat. No. 5,923,456 to D. M. Tench et al. entitled “Reversible Electrochemical Mirror,” issued on Jul. 13, 1999; U.S. Pat. No. 6,111,685 to D. M. Tench et al. entitled “Reversible Electrochemical Mirror (REM) With Improved Electrolyte Solution,” issued on Aug. 29, 2000; U.S. Pat. No. 6,166,847 to D. M. Tench et al. entitled “Reversible Electrochemical Mirror for Modulation of Reflected Radiation,” issued on Dec. 26, 2000; and U.S. Pat. No. 6,400,491 to D. M. Tench et al. entitled “Fast-Switching Reversible Electrochemical Mirror (REM),” issued on Jun. 4, 2002, all incorporated by reference herein. Organic solvents, however, present their own set of problems for reversible mirrors. These problems may include low solubility of charge carriers in organic solvents, poor conductivity and poor solubility of metal ions, low boiling points, toxicity, flammability, low electrochemical stability, low photostability, and poor seal tolerance. Some organic solvents are reduced (and generate hydrogen gas) more easily than the metal ions are electrodeposited. Additionally, organic solvents that are polar enough to support electrochemistry may include functional groups such as ketone and ester groups that may chemically react with metals and interfere with electrodeposition. Another reversible mirror employs a permanent thin metal film that becomes transparent upon exposure to hydrogen gas (see, for example, U.S. Pat. No. 6,535,323 to M. T. Johnson et al. entitled “Light-switching device,” issued on Mar. 18, 2003; U.S. Pat. No. 5,905,590 to P. Van Der Sluis et al. entitled “Optical switching device comprising switchable hydrides,” issued on May 18, 1999; U.S. Patent Application 20020044717 to T. J. Richardson entitled “Electrochromic materials, devices and process of making,” which was published Apr. 18, 2002; and T. J. Richardson et al., “Switchable mirrors based on nickel-magnesium films,” Applied Physics Letters, 2001, 78, 3047-3049, all incorporated by reference herein). These devices operate either by addition and removal of hydrogen gas, which requires gas handling capabilities and the manipulation of highly flammable hydrogen gas, or by electrochemical production of hydrogen gas from highly caustic aqueous solutions
The use of polymer electrolytes formed by organic solvent doped with various lithium salts is restricted in electrochromic devices especially in large area devices due to number of disadvantages associated with them such as poor thermal and low electrochemical stability and hydrophilicity.
This invention relates to the employment of ionic compounds and methods for their preparation. In particular the invention relates to ionic compounds which are liquid at relatively low temperatures, i.e. generally below about 100° C., and preferably below about 60° C. and more preferably which are liquid at or near to room temperature. There is much current interest in the field of ionic liquids. Such systems, which are examples of molten salts, have a number of interesting and useful chemical properties, and have utility, for example, as highly polar solvents for use in preparative chemistry, and as catalysts. They also have particular applications in electrochemistry, for example in batteries, fuel cells, photovoltaic devices and electrodeposition processes, for example in baths for the electroplating of metals. Ionic liquids have very low vapor pressure and thus, in contrast to many conventional solvents, are very advantageous as they produce virtually no hazardous vapors. They are therefore advantageous from a health, safety and environmental point of view.
Ionic liquids, which are designer solvents and called neoteric solvents, have shown that the problems associated with conventional organic solvents, can be overcome by their use. Ionic liquids are typically organic solvents and are composed of ammonium cations and trifluoromethylsulfonyl based anions.
In another embodiment of the present invention the electrolytes may also contain one or more stiffening agents to increase the viscosity of solution while maintaining conductivity. Stiffening agents include, but are not limited to organic monomers such as acrylonitrile, vinlidene fluoride hexafluoropropylene, vinyl alcohol, vinylacetate, methylmethacrylate and their comonomers. These polymers may be formed by in situ polymerization of monomers (for example, U.S. Pat. No. 6,420,036 to D. V. Varaprasad et al. entitled Electrochromic Polymeric Solid Films, Manufacturing Electrochromic Devices Using Such Solid Films, and Processes for Making Such Solid Films and Devices,” incorporated by reference herein).
Meanwhile U.S. Pat. No. 0,257,633 A1 (Agrawal et al) discloses an ionic liquid based device which is free from one or more of the drawbacks such as moisture sensitivity, high volatility, hydrophilicity, low electrochemical and chemical stability and susceptibility to ultraviolet degradation. This patent though descriptive and describes the use of ionic liquids in electrochromic devices, but fails to describe the method for the synthesis of ionic liquids.
Susan et al. (J. Am. Chem. Soc., 127, 2005, 4976 and references there in) discloses a method for the preparation of ionic liquid based on ammonium cations with varying alkyl groups based on Bronsted acid-base combination for use as electrolytes in fuel cells. They also discussed the polymerization of a common monomer in these ionic liquids. Because these electrolytes were made for an application where transparency is not a prerequisite, thereby preventing its use as an electrolytes in electrochromic transmissive devices. Another additional disadvantage is that the electrolyte has some flow property, which acts to reduce the cycle life of the electrochemical cell formed thereof.
This invention embodies large area smart windows as large areas are necessary for sunlight intensity reduction and heat flux control in buildings; the construction of large-scale panels should be achievable in a relatively short time. The new EC windows or devices thus proposed based on an innovative solution, allow the formation of the solid electrolytes, largely based on ionic liquids in a polymer matrix, here after called as ionogels that can be easily produced and sandwiched between two electrodes which is attributable to their good adhesive properties and compatibility with the electrodes.